74 zyxwvutsrqpon / zyxwv CLEO 2001 / MONDAY AFTERNOON zyxwvuts -1 s zyxw tical bandwidth,”Optics Express zyxwvutsrqpon 7, zyxwvutsrqpon 31 1-322 zyxwvutsrqpon (2000). 5. L.J. Gamble, W.M. Diffey, S.T. Cole, R.L. Fork, and D.K. Jones, “Simultaneous mea- surement of group delay and transmission for a one dimensional photonic crystal,”Op- tics Express 5,267-272 (1999). CMP 3:45 pm-5:30 pm Room: 314/315 Ultra Broadband Gereration and Measurements TBA, Presider CMPl 3:45 pm Direct Temporal intensity Measurement of Ultrashort Optical Pulses Using Third-Harmonic-Generatlon based Triple Correlation Tzu-Ming Liu, Yin-Chieh Huang, Gia-Wei Chern, Chih-Jie Lee,* Yu-Chueh Hung,* Chi-Kuang Sun,** Graduate Institute zyxwvutsrqp of Electro- Optical Engineering,National Taiwan University, Taipei 1061 7, TAIWAN R.O.C.; Email:f8941002@ee.ntu.edu.tw; *Department of Electrical Engineering, National Taiwan University, Taipei 10617, TAIWAN R.O.C.; **GraduateInstitute of Electro-Optical Engineering and Department of Electrical Engineering, National Taiwan University, Taipei 10617, TAIWAN R.O.C. Because electronic devices (streak camera etc.) are too slow to measure temporal evolution of ultrashort optical pulses, many techniques were developed to retrieve temporal pulse shape. Most of them extract the temporal intensity either by assuming an analytic pulse shape (autocorrela- tion) or with the help of spectral measurements (including FROG’ and SPIDER.’) AU of these techniques rely on either interference or various nonlinear effect including second harmonic gen- eration and optical Kerr effect. However, in the later 1960 it was shown that a triple correlation is sufficient to determine the temporal intensity of a laser pulse3 with a direct mathematical cal- culation. In this report, we demonstrate direct temporal intensity measurement of ultrashort optical pulses using a novel third-harmonic-gen- eration (THG) based triple correlation method. Using THG process in a single GaN thin film, the optical pulse intensity profile from a mode- locked Cr:forsterite laser was directly obtained with the background free triple correlation trace without any spectral information and pulse- shape assumption. This is different from the pre- vious demonstration4 where triple correlation was obtained through a complex combination of second-harmonic generation and sum-frequen- cy-generation.In order to retrieve the correspon- ding phase information, a simple genetic algo- rithm was also developed for the first time based on the direct optical spectrum measurement with improved O(n) complexity than O(nZ) in.’ An optical pulse with intensity I(t) can be ex- panded in frequency domain v as as shown in Fig. 1. By varying the time delay be- tween pulses t, and t2 and recording the selected THG signals with a detector, the background- free triple correlation I(t) = 1 I(v)exp(-i2xvt)dv = Ij(v)lexp(ia(v) - i2xvt)dv (1) G3 (q,tZ) = zyxw I I(t)Z(t+ q)l(t + t,)dt (2) was directly measured. Then we can use bispec- trum G 3 (vI,vZ), the Fourier transform of triple correlation function, to calculate li(v)l and a(v),’ and thus determine the I(t) using equation (1). Figure 2 shows two examples of the obtained triple correlation traces of ultrashort laser pulses where Il(v)l and a(v) are magnitude and phase of the optical pulse intensity I(t) in the spectral do- main. After beam splitting, three mutual parallel and equal distance laser pulses were focused on the THG crystal and THG signal with three fun- damental frequency photon contributed from three individual pulses was spatially selected with an iris according to momentum conservation law n w CMPl Fig. 1. Diagrammatic representation of THG based triple correlation. --- . 300 -200 -100 0 100 200 300 Delay 11 (ps) Delay ~l (fs) CMPl laser. Fig. 2. Two THG triple correlation traces of optical pulses from a modelocked Cr:forsterite 1 1 z 0.0 U N -0 0.0 L . I -6 - 4 - 3 - 2 - 1 0 1 2 3 -600 -400 -200 0 200 400 600 Delay (PS) Delay (fs) CMPl Fig. 3. relation traces in Fig. 2. Recovered temporal intensity and phase profile corresponding to the THG triple cor-